Conventionally, modern internal combustion engines used in motor vehicles have a fuel supply tank and a checking device for monitoring and, if necessary, preventing the emission of fuel vapors formed in the fuel supply tank. The checking device in particular captures any fuel vapor that occurs via an activated carbon trap, i.e., an activated carbon filter, and stores it temporarily in the activated carbon trap. Volatile fuel vapors, i.e., usually hydrocarbon vapors, are formed, for example, during the operation of filling the tank of the vehicle or because of a rise in the fuel temperature in the tank and the associated rise in the fuel vapor pressure.
The storage capacity of the activated carbon trap declines steadily with an increasing amount of stored hydrocarbon and therefore the activated carbon trap must be regenerated occasionally, i.e., the stored hydrocarbon must be dissolved back out of it. To this end, the activated carbon trap is connected to the internal combustion engine via a fuel-tank venting valve (TEV) having an intake manifold which is used to draw in combustion air via a throttle valve. Opening the fuel-tank venting valve results in a pressure gradient between the activated carbon trap and the intake manifold by which the hydrocarbon stored in the activated carbon trap is sent to the intake manifold to ultimately be burned in the internal combustion engine and thus eliminated.
In the present context, reference is made to the statutory regulations concerning the operation of internal combustion engines, which have been made more stringent in some countries such as the United States; according to these regulations, motor vehicles using volatile fuels such as gasoline must have a checking device as mentioned in the preamble capable of detecting any leakage through an opening 0.5 mm in size in the tank or throughout the entire fuel tank system, using only onboard means.
This regeneration of the activated carbon trap depends in a sensitive manner on the functioning of the fuel-tank venting valve. There is thus a requirement that the fuel-tank venting valve must be checked regularly for proper functioning. A conventional approach for diagnosing the fuel-tank venting valve involves operating the fuel-tank venting valve at a sufficiently stable operating point when idling and observing the change in the composition of the mixture supplied to the internal combustion engine as well as the change in the energy flow through the throttle valve. This energy flow corresponds to the product of the air mass flow removed through the throttle valve and the firing angle efficiency. Therefore, this method presupposes a high intake manifold vacuum.
According to another conventional approach, the diagnosis is performed as part of a conventional leakage test on the tank system. Such a method is described in U.S. Pat. Nos. 5,349,935, 5,890,474, 6,131,550, and 5,898,103, and German Patent Application Nos. DE 196 36 431.0, DE 198 09 384.5 and DE 196 25 702, for example. In these patents, the tank system is acted upon by an excess pressure via a pump and, if applicable, the presence of a leak is determined by subsequent analysis of the pressure variation. Moreover, similar methods are described in Japanese Patent No. 6173837 and U.S. Pat. No. 5,347,971, in which a reference leak is connected in parallel to the tank system, and in which a conclusion is drawn regarding the presence of a leak by comparing the measurements with and without the reference leak. In addition, U.S. Pat. No. 5,890,474 and German Patent Application No. 196 36 431.0 describes that a performance quantity of the pump, e.g., the electric power consumption, may be used in the leakage test. For a function diagnosis of the fuel-tank venting valve, first a check is usually performed on the basis of the reference leak method described above to determine whether the tank system is tight. Assuming the tank is tight, the fuel-tank venting valve is triggered to open. Then if there is a significant drop in the pump's power consumption, the fuel-tank venting valve is assumed to be functioning properly.
These procedures for the function diagnosis of the fuel-tank venting valve require time-consuming measurements and do not allow a quantitative statement with regard to proper functioning of the fuel-tank venting valve.